Giving MatCap_TextureAdd a black outline

I'm trying to give the MatCap_TextureAdd shader a black outline. I think half4 frag(v2f i) :COLOR { return i.color; } is what creates the outline. So I tried copying and pasting it into MatCap_TextureAdd's subshader. But no matter where I put it either I get an error or the outline doesn't appear at all. Is there a specific place within the subshader it needs to go or is there code already using something that half4 frag(v2f i) :COLOR { return i.color; } needs?

 Shader "MatCap/Vertex/Textured Add"
 {
     Properties
     {
         _MainTex ("Base (RGB)", 2D) = "white" {}
         _MatCap ("MatCap (RGB)", 2D) = "white" {}
     }
     
     Subshader
     {
         Tags { "RenderType"="Opaque" }
         
         Pass
         {
             Tags { "LightMode" = "Always" }
             
             CGPROGRAM
                 #pragma vertex vert
                 #pragma fragment frag
                 #pragma fragmentoption ARB_precision_hint_fastest
                 #include "UnityCG.cginc"
                 
                 struct v2f
                 {
                     float4 pos    : SV_POSITION;
                     float2 uv     : TEXCOORD0;
                     float2 cap    : TEXCOORD1;
                 };
                 
                 uniform float4 _MainTex_ST;
                 
                 v2f vert (appdata_base v)
                 {
                     v2f o;
                     o.pos = mul (UNITY_MATRIX_MVP, v.vertex);
                     o.uv = TRANSFORM_TEX(v.texcoord, _MainTex);
                     
                     half2 capCoord;
                     capCoord.x = dot(UNITY_MATRIX_IT_MV[0].xyz,v.normal);
                     capCoord.y = dot(UNITY_MATRIX_IT_MV[1].xyz,v.normal);
                     o.cap = capCoord * 0.5 + 0.5;
                     
                     return o;
                 }
                 
                 uniform sampler2D _MainTex;
                 uniform sampler2D _MatCap;
                 
                 fixed4 frag (v2f i) : COLOR
                 {
                     fixed4 tex = tex2D(_MainTex, i.uv);
                     fixed4 mc = tex2D(_MatCap, i.cap);
                     
                     return (tex + (mc*2.0)-1.0);
                 }
             ENDCG
         }
     }
     
     Fallback "VertexLit"
 }